If the current palette is not yet equal to the target palette, this
function will make a few small changes to the current palette to make
it slightly more like the target. Over time, the current palette will
come to be equal to the target.

There’s no need to test the current and target for equality; it’s safe
to keep calling nblendPaletteTowardPalette even after current reaches target.
For faster blending, call nblendPaletteTowardPalette twice per loop.